Atmospheric Sciences & Global Change Research Highlights

Pollution Stifles Wind, Squelches Rain

Summer air pollution hangs like a dreary curtain around China’s popular Mt. Hua, the site of landmarks such as this tea house. It’s also a popular spot for climbers. The researchers found that air pollution works to suppress the necessary rainfall in the region during certain summer months. Photo courtesy of Jennifer Mo. Enlarge Image.

Results: Like a high stakes match of rock-paper-scissors, summertime pollution
trumps both wind and precipitation in the valley air near Mt. Hua. Scientists at Pacific Northwest National Laboratory
found that human-caused air pollution suppresses up to 40% of the precipitation
over Mt. Hua during one month in the summer.

Normally, precipitation in mountainous areas is strongly influenced by
air coming from up-wind. In China, this air often occurs in a low elevation
plain containing severe air pollution. The researchers found that a major force
responsible for the decline of mountain precipitation is the heavy pollution
that interacts with sunlight energy (solar radiation) to weaken the valley
breeze and reduce surface moisture.

"As a result, the usual transport of water vapor from the valley to the mountain
is significantly reduced," said the study's lead climate modeler Dr. Jiwen Fan, a PNNL atmospheric
scientist. "This causes cascading effects that suppress convection—the process
that forms stormy rain clouds—and precipitation over the mountain."

Why It Matters: Precipitation over mountain ranges is especially important
since it provides a major source of water supply for agricultural, domestic,
and industrial use. Significant reduction in precipitation in the past decades
has been documented over many mountain ranges such as those in central and
eastern China. A major concern
is how the severe pollution in the populated plains affects the downwind mountainous
precipitation. Increased
air pollution in those regions is the likely culprit, because the tiny aerosol
particles are linked with reduced precipitation.

Up until now, there have
been no rigorous studies that quantify the mechanisms responsible for the
precipitation reduction. The PNNL research team discovered a major mechanism
clearly showing how the absorbing aerosol particles in the plain contribute to
the reduced precipitation over the mountain. The study and the finding are not
only important to improve understanding of the weather and climate impacts of
the severe pollution in China, but also useful to inform policies to address emissions
from industrial and automotive sources and air quality.

Methods: The research team led by
PNNL employed
an improved Weather Research and Forecasting (WRF) model with online coupled
chemistry (WRF-Chem), and conducted one-month summer convection simulations at
the convection-permitting scale using current emission data.
To explore aerosol impacts and the specific mechanisms in this study, the
researchers carried out sensitivity simulations by reducing the emissions to
the level before the Chinese economic boom and by turning on and off aerosol-radiative
interactions. They gathered observational data in surface radiation fluxes,
aerosol optical depth, and precipitation to evaluate the model simulations with
current emission data.